[Idea/Maybe WIP] Tower Defense

Is this a good idea?

Yes

No

For a while now, I've been thinking about creating a tower defense type side game as a mod. You would use an item that would generate a mini-world somewhere in the sky that is open. You would then fight off sequentially harder enemies with custom AI to try to get to your castle/tower. The enemies would either drop weapons, or some special type of currency used to buy weapons.